A Hydrobot is a Non-Humanoid Hunter Killer unit produced by Skynet sometime before 2018. It features a serpent-like structure with a claw at their head. It is used to patrol the waters. The Hydrobot is based on the present day model of Skynet Research's Aquatic WorkBot, which was used to maintain kelp and seaweed, find minerals and oil, and seek out organic matter. As the name implies, a Hydrobot has difficulty operating on land, but they could jump incredible distances and use their metal teeth to cut through even metal.
This is one of two hero Hydrobots, this one being used in John Connor's (Christian Bale) base lab and a key prop to the film, as it allows the team to find a "backdoor" to hack into Skynet's code to control the machines. Used in the sequence where the human resistance captures this Hydrobot and field tests the shortwave signal transmitter, successfully deactivating it (on the picture, you can also see the 1/6 replica version made by Hot Toys for their licensed John Connor Figure replica sitting on top of the screen used Hydrobot).
Constructed of aluminum and steel armature and cable actuations for the grapplers and rubber components which are finished to look like metal. The grapplers are pneumatically articulated at the ends. A pair of LEDs provide illumination for the eyes. Inner parts include a rotary gear system for the rubber driller (which resembles steel actually!) and an internal compressed air line system that was probably used by the FX team to "stimulate" the Hydrobot while fighting against the humans trying to deactivate it.
Easy to screenmatch to the whole lab scene thanks to the unique weathering patterns found on both the claws and side parts, plus a tiny broken part on the upper claw.
On the making off we also learn these were build strong solid to resist "heavy" handling while on set.
This new deadly machine is also referenced in the Sarah Connor Chronicles ("menace will come from the waters") and on the T4 posters and promo materials. Plus it has its own chapter on the "Art of Terminator Salvation" book, and its own score on the Original Soundtrack composed by world acclaimed Danny Elfman.
Was built at Legacy Effects Studios (formerly Stan Winston Studios, who passed away while Terminator Salvation was in the works), you can access a picture gallery of its pieces and making off here.
COA : Legacy Effects (formerly Stan Winston Studios)